1. Field of the Invention
The present invention generally relates to lithium ion batteries, more specifically to reducing interfacial lamination resistances of the interface between a lithium ion battery's electrode and electrolyte.
2. Description of the Relevant Art
The three primary functional components of a lithium-ion battery are the anode, cathode, and electrolyte, for which a variety of materials may be used. Typically, the anode of a conventional Li-ion cell is made from carbon, the cathode is a metal oxide, and the electrolyte is a lithium salt in an organic solvent. Commercially, the most popular material for the anode is graphite. The cathode is generally one of three materials: a layered oxide, such as lithium cobalt oxide, a phosphate, such as lithium iron phosphate, or a spinel, such as lithium manganese oxide. Both the anode and cathode are materials into which lithium inserts and extracts. The process of lithium moving into the anode or cathode is referred to as insertion, and the reverse process, in which lithium moves out of the anode or cathode is referred to as extraction. When discharging the cell, the lithium is extracted from the anode and inserted into the cathode. When charging the cell, the exact reverse process occurs: lithium is extracted from the cathode and inserted into the anode.
Liquid electrolytes in Li-ion batteries consist of solid lithium-salt electrolytes, such as LiPF6, LiBF4, or LiClO4, and organic solvents, such as ether. Typically, electrolyte is sandwiched between the interface material of the cathode and anode, and a 25 μm thick porous polypropylene separator interspersed within the electrolyte material. A liquid electrolyte conducts Li ions, which act as a carrier between the cathode and the anode when a battery passes an electric current through an external circuit. However, solid electrolytes and organic solvents are easily decomposed on anodes during charging, thus preventing battery activation. Nevertheless, when appropriate organic solvents are used for electrolytes, the electrolytes are decomposed and form a solid electrolyte interface at first charge that is electrically insulating and high Li-ion conducting. The interface prevents decomposition of the electrolyte after the second charge.
Traditional lithium ion batteries are characterized by dramatically lower capacities (in amp-hours) at high discharge rates (e.g. 20 C, where C is the one hour discharge rate). It is believed that the lowered discharge capacities of the prior art would be alleviated by incorporation of higher amounts of acetylene black (which has relatively higher conductivity per particle), and relatively lower amounts of binder material (e.g. PVDF, or polyvinylidene difluoride). These techniques have not resulted in dramatic changes in lithium ion cell performance increases.
Typically, battery discharge rate versus capacity in amp hours is related by the Peukert equation. Peukert's Law, presented by the German scientist W. Peukert in 1897, expresses the capacity of a battery in terms of the rate at which it is discharged. As the rate increases, the battery's capacity decreases, although its actual capacity tends to remain fairly constant.
Peukert's law is Cp=Ikt, where:                Cp is the capacity according to Peukert, at a one-ampere discharge rate, expressed in amp-hours (A·h);        I is the discharge current, expressed in amps;        k is a dimensionless Peukert constant; and        t is the time of discharge, expressed in hours (h).        
Rather than rate batteries by their one hour discharge rate, manufacturers rate the capacity of a battery with reference to a specified discharge time. Therefore, a modified equation should be used:
      t    =          H                        (                      IH            C                    )                k              ,where H is the hour rating that the battery is specified against and C is the rated capacity at that discharge rate. In this modified equation, Cp no longer appears. In fact, to more closely describe battery performance versus discharge rates, battery curves are commonly provided detailing the amount (depth) of discharge for a given battery versus time with curves depicting different discharge rates. These charts are referred to as rate curves, discharge curves or rate charts.
For an ideal battery, the constant k would equal 1, in which case the actual battery capacity would be independent of the current discharge rate. For a lead-acid battery, the value is typically between 1.1 and 1.3. It should be noted that the Peukert constant varies according to the age of the battery, and generally increases with age.
The Peukert equation becomes a very important issue in an electric vehicle battery where (typically lithium ion) batteries, initially rated at 20 hours, are instead used at much higher rates, typically depleting the batteries on the order of about 1 hour.
What is thus still needed in the lithium ion battery art is a battery exhibiting a capacity at higher discharge rates that is closer to one with a low discharge rate, or alternatively, with a Peukert constant closer to 1.